A radio link for a digital radio transmission system having spatial and/or angular diversity is optimized in real time by searching for a minimum of the analog BER function BERn(φ), by analyzing the minimum to determine if it is an "absolute" minimum that will result in an optimal radio link, and if...http://www.google.com/patents/US5581583?utm_source=gb-gplus-sharePatent US5581583 - Optimizing the analog BER function in a spatial angular diversity digital radio receiver

Optimizing the analog BER function in a spatial angular diversity digital radio receiverUS 5581583 A

Abstract

A radio link for a digital radio transmission system having spatial and/or angular diversity is optimized in real time by searching for a minimum of the analog BER function BERn(φ), by analyzing the minimum to determine if it is an "absolute" minimum that will result in an optimal radio link, and if not, searching for another minimum until the absolute minimum is located. Once the relative phase φ corresponding to the absolute minimum has been thus determined with both channels having the same nominal attenuation level, the relative attenuation level T of the two channels may then be varied in order to optimize the dispersion Bann (T) of the recombined data spectrum while holding the relative phase at its previously optimized value.

Images(12)

Claims(5)

We claim:

1. Method of optimally combining at least two diverse input signals into a recombined data spectrum under varying transmission conditions in a radio link of a diversity reception digital radio transmission system, comprising the steps:

measuring power and dispersion of the recombined data spectrum;

calculating an estimated bit error rate from the power and dispersion thus measured using an analog bit error rate function having predetermined power and dispersion coefficients;

determining whether the thus calculated estimated bit error rate varies by at least a predetermined amount in response to a predetermined change in the relative phase of the at least two input signals;

if the thus determined change is less than the predetermined amount, performing the calculating step with an alternative analog bit error function having different power and dispersion coefficients;

varying a relative phase of the at least two input signals while monitoring the thus calculated estimated bit error rate to thereby find a local minimum of the analog bit error rate function;

analyzing the local minimum thus found to determine if the local minimum is an absolute minimum; and

continuing to vary the relative phase until the absolute minimum is found.

2. Method according to claim 1, further comprising the steps:

determining whether the dispersion passes through a point of minimum dispersion while the change in the estimated analog bit error rate remains below a first predetermined amount during a first predetermined change in the relative phase;

if the dispersion is thus determined to pass through a point of minimum dispersion while the change in the estimated analog bit error rate remains below a first predetermined amount during a first predetermined change in the relative phase, using a first alternate analog bit error rate function with a higher dispersion coefficient; and

if the dispersion is not thus determined to pass through a point of minimum dispersion or if the change in the estimated analog bit error rate does not remain below a first predetermined amount during a first predetermined change in the relative phase,

determining whether the change in the estimated bit error rate remains below a second predetermined amount during a second predetermined change in the relative phase greater than the first predetermined change, and

if the change in the estimated bit error rate is thus determined to remain below a second predetermined amount during a second predetermined change in the relative phase greater than the first predetermined change, using a second alternate analog bit error rate function with a higher power coefficient.

3. Method of optimally combining at least two diverse input signals into a recombined data spectrum under varying transmission conditions in a radio link of a diversity reception digital radio transmission system comprising the steps:

measuring power and dispersion of the recombined data spectrum;

calculating an estimated bit error rate from the power and dispersion thus measured, using an analog bit error rate function having predetermined power and dispersion coefficients;

varying a relative phase of the at least two input signals while monitoring the thus calculated estimated bit error rate to thereby locate a local minimum of the analog bit error rate function;

analyzing the local minimum thus found to determine if the local minimum is an absolute minimum;

continuing to vary the relative phase until the absolute minimum is found;

holding the relative obese fixed once the absolute minimum has been found;

after the absolute minimum has been found, varying a relative attenuation level of the two input signals between first and second predetermined values until an optimal value of the relative attenuation has been found, and

using a dispersion function responsive to the dispersion of the recombined data spectrum but not responsive to the power of the recombined data spectrum to locate the optimal value of the relative attenuation.

4. Method of optimally combining at least two diverse input signals into a recombined data spectrum under varying transmission conditions in a radio link of a diversity reception digital radio transmission system, comprising the steps:

measuring power and dispersion of the recombined data spectrum;

calculating an estimated bit error rate from the power and dispersion thus measured, using a bit error rate function having predetermined power and dispersion coefficients;

determining whether the estimated bit error rate varies by at least a predetermined amount in response to a predetermined change in the relative phase of the at least two input signals;

if the thus determined change is less than a predetermined amount, performing the calculating step using an alternative bit error function having different power and dispersion coefficients; and

varying a relative phase of the at least two input signals while monitoring the thus calculated estimated bit error rate to thereby locate a minimum of the analog bit error rate function.

5. Method according to claim 4, wherein the calculating step further comprises the steps:

determining whether the dispersion passes through a point of minimum dispersion while the change in the estimated analog bit error rate remains below a first predetermined amount during a first predetermined change in the relative phase;

if the dispersion is thus determined to pass through a point of minimum dispersion while the change in the estimated analog bit error rate remains below a first predetermined amount during a first predetermined change in the relative phase, using a first alternate analog bit error rate function with a higher dispersion coefficient; and

if the dispersion is not thus determined to pass through a point of minimum dispersion or if the change in the estimated analog bit error rate does not remain below a first predetermined amount during a first predetermined change in the relative phase,

determining whether the change in the estimated bit error rate remains below a second predetermined amount during a second predetermined change in the relative phase greater than the first predetermined change, and

if the change in the estimated bit error rate is thus determined to remain below a second predetermined amount during a second predetermined change in the relative phase greater than the first predetermined change, using a second alternate analog bit error rate function with a higher power coefficient.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 08/066,298 filed May 21, 1993 now abandoned.

Other related inventions are disclosed and claimed in our commonly assigned U.S. patent applications filed on 25 May 1993 under Ser. No. 08/066,386 (claiming priority from Italian application MI92 A 001270) and 08/66,387 (claiming priority from Italian application MI92 A 001269 and now abandoned).

FIELD OF THE INVENTION

The present invention relates to a spatial and/or angular diversity digital radio transmission system, and more particularly to optimizing the combining of the diverse input signals in such a system under varying transmission conditions.

BACKGROUND ART

In commonly assigned published Italian Patent No 1,227,559, there is described a system for combining at least two signals which are received at different locations in space (spatial diversity reception) and/or at different angular orientations (angular diversity reception); FIG. 1 of that publication is a generalized schematic of such a diversity combining system. In the known diversity receiver, an intermediate-frequency estimated BER (bit error rate) function is optimized in order to provide a minimum-BER combiner.

The intermediate frequency estimated BER function is called "analog BER" and is given by the formula:

BER=10.sup.αP+β +10.sup.γD+δ ( 1)

In this formula the variables P and D correspond to the power and dispersion of the combined signal, while the coefficients α, β, γ, δ depend on the type of modulator-demodulator employed.

It is possible to re-express equation (1) in the form:

BER=A·Rag(P)+B·Ban(D) (2)

where:

Rag(P)=10.sup.αP

Ban(D)=10.sup.γD

A=10.sup.β

B=10.sup.δ

in which the function Rag(P) represents the power of the recombined data spectrum, and the function Ban(D) represents the dispersion of said spectrum.

Since power and dispersion depend on the recombination phase φ and the attenuation T1, T2 introduced on each of the two channels by respective attenuators, the analog BER function for any particular combination of input signals on the two channels can also be expressed as:

BER(φ,T1,T2)

it being understood that the analog BER function is also dependent on the characteristics of the two input signals being applied to the combiner.

The channel that both phase-shifts and attenuates the signal upstream of the summing node may be designated MAIN and the channel that only attenuates upstream of the node may be designated DIV, as shown in FIG. 6 of the above cited Italian Patent 1,227,559 herein reproduced as FIG. 1. In this FIG the references indicate respectively:

MAIN: channel that phase-shifts and attenuates the signal upstream of the summing node.

DIV: channel that attenuates upstream of the summing node.

8: driven attenuator T1

9: driven attenuator T2

10: delay line

11: driven phase shift

12: summing node

13: IF amplifier

14: power detection filter

15: detector

16: automatic gain control

17: dispersion measurement network

18: A/D converter

19: A/D converter

20: microprocessor

21: D/A converter

22: D/A converter

23: D/A converter

Assuming a dispersion on the MAIN channel characterized by an echo delay τ, a notch depth Bcm and a notch frequency position Fnm, and also assuming a dispersion on the DIV channel characterized by an echo delay τ, a notch depth Bcd and notch frequency position Fnd, these parameters will define the selective fading present on the MAIN and on the DIV channels, resulting in a first analog BER function BER1 (φ,T1,T2).

As the parameters determining the selective fading present on the two input channels change, one will obtain other analog BER functions BERn (φ,T1,T2) different from BER1 (φ,T1,T2). Therefore it will be possible to define countless BERn (φ,T1,T2)'s corresponding to the countless possible selective fading conditions.

Setting T1 and T2 equal to the same nominal attenuation value (T1n indicates a nominal value of T1 and T2n indicates a nominal value of T2) results in the function:

BERn (φ)=A·Ragn (φ)+B·Bann (φ)(3)

For optimizing BERn (φ), varying φ in accordance with a conventional gradient search technique is usually sufficient to locate the desired minimum, assuming that the BERn (φ) function was reasonably smooth.

However, even assuming that particular BERn (φ) was a continuous function, a potential difficulty in optimizing BERn (φ) is that certain functions BERn (φ) may be constant over a phase interval Φ0<φ<Φ1 in which BERn (φ) has a minimum.

It will be understood that the phase shift present on the MAIN channel is continuously varied in order to optimize as quickly as possible the selective fading parameters present on the two channels. This usually results in a relatively restricted phase oscillation in the neighborhood of the optimum value of φ which allows BERn(φ) to be maintained at a minimum without significantly degrading the combined signal data spectrum. However, in the case of a BERn (φ) which remains constant over a relatively large phase interval containing the desired minimum, conventual gradient techniques to locate that minimum will result in a large excursion of φ, and may thereby cause excessive variations in the dispersion and power of the recombined signal. This possibility exists because even though BERn (φ) is held relatively constant over a large variation of φ, a resultant large variation in the dispersion Bann (φ) may be offset by a complementary large variation in the power Ragn (φ), provided that:

BERn (φ)=B·Bann φ+A·Ragn (φ)≅constant (4)

The problem is even more significant because such excessive variations in dispersion and power may result in an oscillation of the recombined signal data spectrum.

Accordingly, a BERn (φ) which remains constant over a relatively large phase interval is not necessarily suitable for determining the optimum phase for combining the two input signals.

Another problem arising during optimization of BERn (φ) function, is that in general the BERn (φ) function will not have only one minimum but will have both a relative minimum Mrel and an absolute minimum Mass (FIG. 3A). The two minimums are not equivalent; in fact there are BERn (φ)'s that have a relative minimum corresponding to an "out of order" radio link, and an absolute minimum for which the link performs satisfactorily.

DISCLOSURE OF INVENTION

It is an overall object of the present invention to provide an improved method of processing and optimizing the analog BER (bit error rate) function in the receiver of a spatial and/or angular diversity digital radio transmission system having two or more inputs, to thereby provide an optimum combining of the inputs.

According to one aspect of the invention, the radio link is optimized by searching for a minimum of the analog BER function BERn (φ)=A·Ragn (φ)+B·Bann (φ) in which A and B are power and dispersion coefficients respectively; analyzing the minimum to determine if it is only a "relative" minimum not corresponding to the best condition for the link; and continuing the search until an "absolute" minimum is reached that corresponds to the best condition for the link.

According to another aspect of the invention, if a situation is encountered in which the normal estimated BER function does not change significantly as the phase φ is varied, an alternate estimated BER function will be used having different weights for the power and dispersion components, until the normal estimated BER function is once again suitable for use in optimizing the radio link.

In a preferred embodiment, the phase is first optimized as outlined above, and then the dispersion is used to determine an optimum attenuation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a generalized schematic of a prior art combining system and corresponds to FIG. 6 of Italian Patent 1,227,559;

FIG. 2 shows a BERn (φ) where the dispersion minimum Md is well defined while the BERn (φ) is almost constant;

FIG. 3 shows a BERn (φ) for which the power has a weight higher than dispersion and in which before observing a variation of the power it is necessary to vary the phase by several degrees;

FIG. 3A shows a BERn (φ) which has both a relative minimum and an absolute minimum;

FIG. 4 is a block diagram of an optimization function that in most cases will be equal to BERn (φ) but that in some instances will be equal to BER1n (φ) or to BER2n (φ);

FIG. 5 is another block diagram showing how an absolute minimum is identified;

FIG. 6 is a simplified vectorial representation of a radio transmissive channel showing how a direct ray and a reflected ray may interact to provide selective fading;

FIG. 7A shows the direct and reflected rays received on one channel;

FIG. 7B shows the direct and reflected rays received on another channel;

FIG. 7C shows a first recombination phase maximizing the power;

FIG. 7D shows a second recombination phase minimizing dispersion;

FIG. 7E shows a third recombination phase also minimizing dispersion;

FIG. 8 is a block diagram for checking whether a relative minimum is an absolute minimum;

FIG. 9 shows the use of a signature measurement to evaluate whether the channels have been combined in an optimum manner;

FIG. 10 shows a modification to FIG. 1 wherein a single control voltage controls both attenuators;

FIG. 11 is a block diagram for a generalized method for varying both the phase and the attenuation in order to optimize the analog BER;

FIG. 13 is a block diagram of a preferred implementation of the generalized method of FIG. 11; and

FIG. 14 is a generalized schematic of a preferred embodiment of a system for implementing the above methods.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Let BERkn (φ) be an alternate BER estimating function that is obtained by a different weighting of the dispersion and power components of equation (3); such an alternate function gives higher weight to dispersion or to power as the values of multiplicative coefficients A and B are varied.

It is necessary to decide in what cases, with BERn (φ) (curve I° in FIGS. 2 & 3) being constant, the function to be optimized will be BER1n (φ) in which the power function Bann (φ) will have a weight greater than the dispersion function Ragn (φ), and in what cases it will be BER2n (φ) in which Ragn (φ) has a weight greater than Bann (φ).

In cases where the function to be optimized is BER1n (φ), the situation will be similar to the one shown in FIG. 2 in which the dispersion minimum Md of the function Bann (φ) (curve II°) is well defined, and the function is no longer at its minimum after only a slight variation of φ (eg, only a few degrees), while BERn (φ) (curve I°) remains almost constant over a much larger range of φ (eg, some tens of degrees).

In order to identify such a situation with certainty it is necessary that at least the following conditions occur:

1. Movement of the phase shift several degrees in the same direction (i.e. shifting the phase n1 times in the same direction) will result in the dispersion Bann (φ) being subjected to a substantial variation, while BERn (φ) does not vary much (let Difb be this minimum variation of BERn (φ)).

2. The dispersion of the recombined data spectrum will increase once the optimum dispersion has been arrived at, even though BERn (φ) remains nearly constant; in practice referring to FIG. 2 one must have passed through the minimum dispersion point (Md) before the function BER1n (φ) can be considered optimized.

In cases when the function to be optimized is a BER2n (φ) in which the power Ragn (φ) has a weight higher than Bann (φ) there will be a situation like that of FIG. 3, in which it can be seen that before observing a substantial variation of the power Ragn (φ) (curve III°) in the region where BERn (φ) remains nearly constant (let Difr be this minimum variation of BERn (φ)) it is necessary to vary φ by several degrees (i.e. shifting the phase n2 times in the same direction, n1<n2). Moreover in this case one does not pass through a zero dispersion point, so that although both the power and the dispersion will vary in response to variations in φ, the dispersion will remain at a relatively high value.

In both cases, as soon as BERn (φ) begins varying again as φ varies (e.g. because the notches present on the two channels have moved away in frequency), the function to be optimized should again be the original BERn (φ) function.

The search of the best combination phase φ thus entails the optimization of a function (herein designated as Funz) that in most of the cases will be equal to BERn (φ) but that in some particular instances will be equal to BER1n (φ) or to BER2n (φ) (block diagram of FIG. 4).

86: evaluates whether after the phase shift Funz(φ) is worsened and, in the affirmative, it inverts the shifting direction of phase (φ).

As noted previously, another difficulty arising during optimization of the BERn (φ) function, is that as a general rule this function does not have only one minimum but has at least a relative minimum Mrel and an absolute minimum Mass (FIG. 3A); not all such minimums will result in the same performance of the link. It is therefore desirable, once a minimum BERn (φ) has been reached, to recognize if that minimum is only a relative minimum that does not optimize the performance of the radio link, and in that case to continue to search for the absolute minimum.

FIG. 5 shows a modified version of the block diagram of FIG. 4, in which the recombined data spectrum is to be subjected to additional tests in order to be able to determine whether or not an absolute minimum has been identified.

As can be seen in FIG. 5, blocks 73 to 86 of FIG. 4 are preferably supplemented by the following blocks:

87 evaluates whether the current phase φ minimizes BERn (φ).

88 checks if the dispersion of the combined data spectrum is greater than value DL after BERn (φ) has reached a minimum.

90 checks if the minimum is an absolute minimum. The function of block 90 is shown in more detail in FIG. 8.

89 carries out a phase shift sufficient to pass over a relative minimum.

By using the known vectorial representation of the radio transmissive channel affected by selective fading (reduced three-ray model), two vectors, one representing the direct ray Rd and the other representing the reflected ray Rr, are obtained (FIG. 6). The reflected ray Rr assumes different phase shifts θi with respect to the direct ray Rd depending on the frequency position of the selective notch (Nse).

Similarly, in a combiner of a spatial and/or angular diversity radio receiver, the two received signals can be characterized in terms of vectors: in FIGS. 7A and 7B there are illustrated the direct ray Rdd received on the DIV channel, the direct ray Rdm received on MAIN channel, and the respective reflected rays Rrd and Rrm.

The absolute value and phase shift of each of the reflected rays Rrd and Rrm of the two channels, relative to the corresponding direct ray, characterize the dispersion due to the selective fading associated with each of the two received signals.

Inspection of FIGS. 7C to 7E shows that there exist at least three possible recombination phases φ1, φ2, φ3: one that maximizes the power (FIG. 7c, φ1=0) and two that reduce the dispersion at the expense of a reduced recombined signal power (FIGS. 7d and 7e). The recombination phase obtained by optimizing BERn (φ) will normally be an optimum phase that results in both low dispersion and high power.

However there are some BERn (φ)'s for which the minima at minimum dispersion and at maximum power do not correspond to recombination phases providing comparable performance for the radio link. Some typical cases are those in which for both channels there is selective fading with a notch coinciding with or close to the band-center frequency of the data spectrum in transmission; in such case the minimum dispersion recombination points therefore practically coincide. For such BERn (φ)'s it is necessary to identify a criterion which facilitates the following two operations:

1 Identifying a non-optimal relative minimum that results in degraded system performance or even an out of order condition.

2 Positioning on the absolute minimum that results in the optimization of the radio link.

A non-optimal relative minimum of BERn (φ) that corresponds to the minimum dispersion can be identified by monitoring the power of the recombined data spectrum.

The case in which the non-optimal relative minimum of BERn (φ) coincides with the maximum power is more difficult to evaluate.

In either case it is necessary to establish if the current phase is the one that minimizes BERn (φ). Once a phase that minimizes BERn (φ) has been reached, the phase shifter will move in the neighborhood of that phase locking itself to the minimum. Therefore in order to determine that the current phase corresponds to a minimum value of BERn (φ), it is sufficient to check whether a predetermined number (ns) of displacements in the neighborhood of the current phase value have already been carried out.

If the data spectrum dispersion (Bann) is greater than a predetermined value DL (88), the identified minimum is one that maximizes the power (FIG. 5). However, this does not necessarily mean that a better minimum exists, since for some BERn (φ)'s the minimum that maximizes power is the absolute minimum. It is therefore necessary to also check for other conditions which permit a better minimum to exist for combination purposes. Such conditions occur when the repective depths Bc of the notches present on the two channels are considerably different, such as to allow an optimal recombination that favors a minimum dispersion.

The following procedure may be used to determine the existence of the above-described conditions (FIG. 8):

1 Attenuators T1 and T2 are unbalanced in one direction, T1 with an attenuation value that exceeds by a certain amount "delta" the attenuation nominal value (T1a =T1n +delta) and T2 with a lower value (T2a =T2n -delta (92); then they are unbalanced in the opposite direction in which T1 has a value lower than the attenuation nominal value by a certain amount "delta" and T2 has a respective higher value (94).

2 Following each such unbalance of the attenuators, the analog BER (93, 95) is checked for a corresponding improvement indicating that the depth Bc of the two notches is considerably different, and that a better recombination favoring a minimum dispersion is therefore possible.

Thus, if such an improvement is detected, the minimum is a relative minimum (97), otherwise the minimum is absolute (96) for BERn (φ).

92: the attenuation level of the two channels is unbalanced in a first direction; the attenuation values after such unbalance will be T1a =T1n +delta and T2a =T2n -delta.

93: the value of BERn (T1n, T2n) stored in block 91 is compared with the value of BERn (T1a, T2a) obtained by carrying out the unbalance of block 92 to check if an improvement of BERn (T1,T2) has occurred.

94: the attenuation level of the two channels is unbalanced in a direction opposite to the one of block 92; the attenuation values after such unbalance will be

T1b =T1n -delta and T2b =T2n +delta

95: the value of BERn (T1n,T2n) stored in block 91 is compared with the value of BERn (T1b,T2b) obtained by carrying out the unbalance of block 94 in order to check if an improvement of BERn (T1,T2) has occurred.

96: both tests of blocks 93 and 95 have not been successful and therefore it can be asserted that the current minimum for BERn (φ) is an absolute minimum.

97: one of the tests of blocks 93,95 has been successful and hence it can be asserted that the current minimum for BERn (φ) is a relative minimum.

In both the examined cases, once it has been determined that the current minimum is a non-optimal relative minimum of BERn (φ), the phase is merely shifted (89) by a sufficient amount to ensure that it will converge to another minimum, and thus that it will eventually converge to the absolute minimum of BERn (FIG. 5).

The above-described method of distinguishing the relative minima of BERn (φ) in which the power is maximized may be validated utilizing a type of "signature" analysis (FIG. 9) which typically is performed on a conventional radio system not provided with spatial and/or angular diversity reception in order to evaluate its susceptibility to selective fading.

The signature analysis is performed on the receiver of the present invention using the following protocol:

1 One of the channels is subjected to a selective fading at the frequency of the radio carrier having a notch depth sufficient to result in a high error rate, e.g. 10 E-6, 10 E-5, . . . , if the system were not provided with a diversity reception channel to mitigate the effects of such selective fading.

2 The selective fading parameters are varied on the other channel both as to frequency position and as to notch depth Bc while monitoring the response of the combined system.

3 The two fading channels are kept in phase with one another.

The result is represented in FIG. 9.

In zone 1 of FIG. 9, a minimum of BERn (φ) that maximizes the power is an absolute minimum, while in zone 2 such a minimum is a relative minimum that is not optimal for radio linkage purposes.

Zone 3, which may represent an out of order area of the system, is a zone where there is no minimum of BERn (φ) more optimal than the one which maximizes the power, since the depth Bc of the notches present on the two channels is not sufficiently different.

Zone 4 is a zone in which minimizing the BERn (φ) through a conventional gradient method does not present any problem.

The above-described problems in optimizing φ arise for some BERn (φ)'s while for others they do not exist, but it should be noted that the solution of such problems is very important to ensure the correct operation of the combiner under conditions that may be encountered in a practical application, i.e. with the parameters of selective fading present on the MAIN and DIV channels varying dynamically and resulting in a rapid succession of different BERn (φ)'s that must each be optimized in real time.

In a preferred embodiment, the analog BER function is optimized not only by varying the phase between the MAIN channel and the DIV channel, but also by varying the attenuation value introduced on the two channels of attenuators T1 and T2 .

Varying the attenuation value of T1 and T2 is advantageous for many reasons; in particular, the two signals even in the absence of any selective fading can reach the combiner with different power levels.

This can occur because of different propagation features characteristic of the two channels, or because of a temporary mispointing of the antennas (in a spatial diversity system).

In such circumstances if the level difference between the two channels is not equalized by means of attenuators T1 and T2, the tolerance of the combiner to selective fading will degrade as a function of the level difference.

Moreover, it may happen that also when levels of the two signals due to flat fading are equal, there are selective fading conditions of the two channels for which, besides optimizing the analog BER function by varying the phase, it is advantageous to vary the attenuation value of T1 and T2 to the end of combination.

The variables are therefore three, but if the two attenuation commands are combined so that if one of the two attenuators increases the attenuation with respect to a nominal value, the other decreases it and vice versa, there is defined a variable T which measures the unbalance between the two attenuators (FIG. 10).

The resultant two variables, corresponding to an attenuation unbalance command (95) and a phase shifter command (96), are therefore the variables to be acted upon to optimize the analog BER function for which the following symbol is adopted: BERn (φ,T). As shown in FIG. 10, T is the value of the attenuator unbalance command (95) to a control circuit (97) which outputs first and second control voltages U1 and U2 between a minimum value Umin and a maximum value Umax in a complementary manner such that when one control voltage is at said minimum value, the other control voltage is at said maximum value, and vice versa.

In the preferred embodiment, the BERn (φ,T) function is first optimized as described above by varying the phase and the attenuator unbalance is then varied about the optimum point obtained with said phase optimization.

Such a method is represented by the block diagram in FIG. 11.

In FIG. 11 the following blocks are represented:

66: the overall function.

67: handles the variation of the phase.

68: evaluates the effects of the phase variation, both ways, for the purpose of BERn optimization.

69: handles the variations of the attenuator unbalance T.

70: evaluates the effects of the variation of the attenuators unbalance T for the purpose of the optimization of BERn (φ).

At this point it is evident that the phase optimization of BERn (φ,T) is predominant with respect to the subsequent optimization carried out by moving the attenuators.

By moving the attenuators it is possible to improve some situations for which the phase optimized BERn (φ) has been already found; in fact, by executing shifts of the attenuators one can reduce the dispersion of the recombined data-spectrum without the risk of drastically lowering the power, an eventuality always present when one tries to minimize dispersion through phase shifting.

One typical circumstance in which it is advantageous to vary the attenuator command is the one of FIG. 12 where a recombined data spectrum which presents a linear dispersion is shown.

By merely optimizing the phase analog BER one does not necessarily obtain a better data spectrum, e.g. because the flat attenuation level of the two signals is different; in this case it is sufficient to equalize such a level difference by varying the value of T to obtain a data spectrum with practically zero dispersion without the occurrence of significant power variations in the recombined spectrum.

Moreover there are some circumstances in which the flat attenuation level of the two signals is equal, and yet the recombined signal data spectrum is still similar to the one of FIG. 12 after having reached the optimum phase for BERn (φ).

In these circumstances varying the attenuation level (T1, T2) while trying to minimize dispersion also results in a recombined data spectrum with practically zero dispersion without significant power variations.

Although in most cases it will be immaterial whether the dispersion Bann (T) or the analog BER is the function being optimized as the attenuator unbalance command is varied, optimizing dispersion is preferred since, the risk under certain conditions of completely turning off one of the channels is reduced.

This would not be desirable because when the conditions change, turning on the channel by resetting its attenuation to its nominal value requires more complexity and hence higher costs.

Therefore it is advantageous to optimize the dispersion Bann (T) when moving the attenuator unbalance command, and moreover it is of advantage to restrict unbalance within certain values which will be designated by Tmin and Tmax (block diagram of FIG. 13, which is a preferred implementation of the generalized block diagram of FIG. 11).

The following blocks are shown in FIG. 13:

66: the overall function.

67: handles variations of phase.

68: evaluates the effects of variation, both ways, of the phase for the purpose of BER(φ) optimization.

69: handles the variations of the attenuators unbalance command T.

71: limits the range of the attenuators unbalance command T between a maximum value Tmax and a minimum one Tmin.

72: evaluates the effects of variation of attenuators unbalance command T for the purpose of Bann (T) optimization.

A preferred embodiment of a system for implementing the above methods will now be described, with reference to FIG. 14.

In FIG. 14, the various blocks respectively indicate:

101, 102: RF amplifier

103, 104: Mixer

105,106,115: Detector

107: Automatic Gain Controller

108: Driven Attenuator T1

109: Driven Attenuator T2

110: Delay Line

111: Driven Phase Shifter

112: Summing node

113: IF Amplifier

114: Power Detection Filter

116: Automatic Gain Control

117: Dispersion Measuring Network

118,119: A/D Converter

120: Microprocessor

121,122,123: D/A Converter

It will be noted that the system of FIG. 14 includes an automatic gain controller (107) for the two channels upstream of the mixers (104 and 103), which controls the two variable gain devices 101,102 (one on the MAIN channel and the other on the DIV channel) placed in the two RF sections upstream of the mixers (103,104), by means of a single control voltage (FIG. 13). The AGC control voltage (107) driving the two devices (101 and 102) is determined by the detecting device (106,105) which receives the higher power level. As a result, if on the MAIN channel there is an incoming signal with a lower power than that on the DIV channel, the AGC voltage which drives both 101 and 102 originates from the detecting device (106) which measures the power of DIV. This solution prevents signals from reaching the mixers with a power greater than the input nominal level for such devices which is one of the reasons for which upstream of the mixer a variable gain device is required; moreover, devices 101 and 102 will have always the same gain thus avoiding the creation of problems during the combination step.

In the interest of clarity, the invention has been described with reference to certain specific and preferred embodiments; countless modifications, substitutions and the like will doubtless be apparent to those skilled in the art without departing from the scope and the spirit of the invention.